1. Field of Invention
The present invention relates to an error compensator for a receiver of multiplexed PSK-type of QAM-type signals which have further been amplitude modulated to provide a synchronization signal for demultiplexing.
2. Description of the Prior Art
Long distance communication channels used for the exchange of digital data are expensive. In order to make efficient use of these channels the rate of communication has been increased to a level that is much higher than the generation rate of any single source of digital data. Therefore the data from several relatively low-speed data sources are frequently combined into a single data stream at the transmitter. The data is then sent as a stream over a communication channel at a relatively high speed. At the receiving end the data stream is separated into its original components for distribution. In order to insure that this separation is done properly a synchronizing signal must be sent along with the data stream by the transmitter to the receiver.
In the co-pending commonly assigned U.S. Pat. Ser. No. 447,988, filed Dec. 8, 1982, incorporated herein by reference, a device was described and claimed in which the data stream was sent in form of PSK or QAM signals. For synchronization, alternate signals are radially amplitude modulated and the change in amplitude is used to derive a channel separation signal. In order to limit the power required to transmit this type of signal and to insure that its signal-to-noise ratio is not unduly degraded the amplitude modulation is done at a very low level compared to the peak amplitudes of the signals. Therefore in order to detect the alternate signals having modulated amplitudes, a plurality of signals are averaged over a period of time to derive said channel separation signal.
It has been found in general that it is important to get rapid, preferably on-line information on the quality and performance of a communication channel being used by measuring the signal-to-noise ratio of the received signals. In the co-pending commonly assigned patent application Ser. No. 509,716 filed June 30, 1983, incorporated herein by reference, a device is disclosed and claimed for measuring on-line the performance of a communication channel by comparing the received PSK or QAM signals with the corresponding nominal or ideal signals as represented by signal constellations mapped in a complex plane. The resulting error signal is used to calculate the impairment of the channel. However when this method of measuring channel performance was applied to the system with radial amplitude modulation for synchronization, it was found that the error signals were distorted and amplified by the radial amplitude modulation. Furthermore it was found that if the receiver equalizer is not adjusted dynamically to take into account radial amplitude modulation, the performance of the receiver is slightly degraded.